Captain Ahab’s Copepods

Baleen whales do not possess teeth but rather filter their food. In spite of their diet of tiny organisms known as plankton, these filter-feeders can still reach a size comparable to that of the massive sperm whale above. Source: Wikimedia Commons

As the Morgan sails offshore from Newport to Vineyard Haven, voyager Peter Norberg stands on the deck, looking out at the expanse of water before him. For Norberg, a Melville scholar who hails from Saint Joseph’s University, this leg of the trip holds great significance. Indeed, the ship is traversing the very same path followed by Captain Ahab in Herman Melville’s Moby-Dick. Unlike these nineteenth-century sailors, however, Norberg has a much more comprehensive understanding of what exactly drove Ahab to search for whales in this area.

For many, much of the ocean seems uninhabited, an empty blue desert that presents periodic appearances of megafauna. As the citizen scientists onboard, 38th Voyagers such as Norberg can no longer plead such ignorance. Within the first few hours of data collection, it quickly became clear that the ocean is in fact inhabited by millions of tiny plants and animals. These organisms can be divided into two categories: phytoplankton (plants) and zooplankton (animals). Phytoplankton harness energy from the sun via the process of photosynthesis, producing oxygen as a byproduct. It is estimated that 40% of the oxygen on earth is produced by these organisms, and every other species in the ocean depends on them directly or indirectly for this reason. Among zooplankton, some are juveniles of large, strong-swimming adults, while others will remain plankton for their entire lives. Because the zooplankton feed on phytoplankton directly, both types live near the top layer of the ocean, where sunlight is readily available for food production.

In spite of their size, plankton are not just a direct food source of other tiny organisms. On the contrary, there are many large species, known as “planktivores,” that constantly feed on this abundant population, including baleen whales. Because of this, understanding the number and diversity of plankton becomes crucial in predicting the location of these whales during the 38th Voyage. At every hourly logbook entry, two different nets are lowered into the water for a five-minute tow if at sea, or a twenty-meter tow at port. The plankton nets differ in mesh size—one designed for capturing tiny phytoplankton, the other for the slightly larger zooplankton. Like the water sample, plankton samples are then analyzed underneath a microscope, and the number of each type of plankton recorded. Voyager Dave Grant dedicated an entire project to examining the presence of copepods, a form of zooplankton that is a key food organism for various species of whale. Not surprisingly, he found an abundance in Stellwagen Bank, where the Morgan encountered three different species of these massive mammals.

A microscope is required to accurately see the copepod fragments and egg masses that make up the diet of many whale species. Source: Dave Grant

Early whalers knew that whales were attracted to concentrations of food that were located according to season. However, without access to a microscope, these sailors likely took a different approach to determining the presence of plankton. They may have observed the larger species that plankton attracted to the area, such as bony fish or even sea birds. Specific interest in plankton as an indicator of wildlife diversity eventually sparked in the 1920s, when marine biologist Alistair Hardy conceived the Continuous Plankton Recorder (CPR) survey as a means of mapping plankton in space and time. This ongoing project has been run for over 80 years, and represents the most extensive long-term survey of marine organisms in the world. It has led to our present understanding of the dynamics of plankton throughout the ocean basins through seasonal and annual cycles. Perhaps more importantly, this study has related plankton changes to more than just biodiversity. As plankton generate a significant amount of oxygen on the planet, they remove carbon dioxide, a greenhouse gas, from the atmosphere. In other words, these miniscule organisms do more than just feed Melville’s monsters. They are key players in helping the earth resist changes to temperature, acidification, and pollution.